Asymmetric Morita−Baylis−Hillman Reaction Catalyzed by Isophoronediamine-Derived Bis(thio)urea Organocatalysts

Enantioselective hydrophosphonylation of N-Boc imines using chiral guanidine-thiourea catalysts

The enantioselective hydrophosphonylation of N-Boc imines was investigated using a new family of pseudo-symmetric guanidine-thiourea catalysts, providing α-amino phosphonates in moderate to high yields with good enantioselectivity. The catalyst was heterogenized by polymerization with styrene and the resulting catalyst was applied to reactions under continuous-flow conditions.

HFIP Mediates a Direct C-C Coupling between Michael Acceptors and Eschenmoser's salt

A direct C-C coupling process merging Michael acceptors and Eschenmoser's salt is presented. Albeit reminiscent of the Morita-Baylis-Hillman reaction, this process requires no Lewis base catalyst. The underlying mechanism could be unveiled by a combination of kinetic, isotopic labelling experiments as well as computational investigations, showcasing the critical role of HFIP as a superior mediator for proton transfer events as well as the decisive role of the halide counterion.

Enantioselective Bifunctional Ammonium Salt-Catalyzed Syntheses of 3-CF3S-, 3-RS-, and 3-F-Substituted Isoindolinones

We herein report the ammonium salt-catalyzed synthesis of chiral 3,3-disubstituted isoindolinones bearing a heteroatom functionality in the 3-position. A broad variety of differently substituted CF3S- and RS-derivatives were obtained with often high enantioselectivities when using Maruoka's bifunctional chiral ammonium salt catalyst. In addition, a first proof-of-concept for the racemic synthesis of the analogous F-containing products was obtained as well, giving access to one of the rare examples of a fairly stable α-F-α-amino acid derivative.

Brucine Diol-Catalyzed Enantioselective Morita-Baylis-Hillman Reaction in the Presence of Brucine N-Oxide

Brucine diol (BD) catalyzed asymmetric Morita–Baylis–Hillman (MBH) reaction is observed for the first time. Brucine N-oxide (BNO) was found to not have an effective chiral catalyst. Faster reaction rate was obtained using unsaturated ester or aromatic aldehydes in the presence of BNO. 4-Nitrobenzaldehyde and α,β-unsaturated ketone/ester were converted to the MBH adduct in moderate yields (up to 74%) with 70% ee value by this catalytic system. The mechanism of BD catalysis is probably initiated by conjugating the vicinal diol of BD to the carbonyl group of the aromatic aldehyde through hydrogen bonding. The tertiary amine of BD acts as a nucleophile to activate vinyl ketone for coupling with the carbonyl of aldehyde through an intramolecular carbonylated reaction. Finally, the breakdown of the complex caused the formation of the MBH adduct (a benzyl-allyl alcohol). The chirality of the benzyl-allyl alcohol is likely affected by the interaction of the bulky asymmetric plane of BD.

Chiral Thioureas-Preparation and Significance in Asymmetric Synthesis and Medicinal Chemistry

For almost 20 years, thioureas have been experiencing a renaissance of interest with the emerged development of asymmetric organocatalysts. Due to their relatively high acidity and strong hydrogen bond donor capability, they differ significantly from ureas and offer, appropriately modified, great potential as organocatalysts, chelators, drug candidates, etc. The review focuses on the family of chiral thioureas, presenting an overview of the current state of knowledge on their synthesis and selected applications in stereoselective synthesis and drug development.

Bifunctional Molecular Photoswitches Based on Overcrowded Alkenes for Dynamic Control of Catalytic Activity in Michael Addition Reactions

The emerging field of artificial photoswitchable catalysis has recently shown striking examples of functional light-responsive systems allowing for dynamic control of activity and selectivity in organocatalysis and metal-catalysed transformations. While our group has already disclosed systems featuring first generation molecular motors as the switchable central core, a design based on second generation molecular motors is lacking. Here, the syntheses of two bifunctionalised molecular switches based on a photoresponsive tetrasubstituted alkene core are reported. They feature a thiourea substituent as hydrogen-donor moiety in the upper half and a basic dimethylamine group in the lower half. This combination of functional groups offers the possibility for application of these molecules in photoswitchable catalytic processes. The light-responsive central cores were synthesized by a Barton-Kellogg coupling of the prefunctionalized upper and lower halves. Derivatization using Buchwald-Hartwig amination and subsequent introduction of the thiourea substituent afforded the target compounds. Control of catalytic activity in the Michael addition reaction between (E)-3-bromo-β-nitrostyrene and 2,4-pentanedione is achieved upon irradiation of stable-(E) and stable-(Z) isomers of the bifunctional catalyst 1. Both isomers display a decrease in catalytic activity upon irradiation to the metastable state, providing systems with the potential to be applied as ON/OFF catalytic photoswitches.

Dynamic control over catalytic function using responsive bisthiourea catalysts

Responsive molecular motor-based bisthiourea organocatalysts were used in the enantioselective Henry reaction to achieve efficient dual stereocontrol in the presence of an external base.

Cooperative Trifunctional Organocatalysts for Proficient Proton Transfer Reactions

Cooperativity is essential to proficient catalysis, and designing biomimetic, cooperative catalysis is a major avenue to finding new and efficient chemical reactions with practical applications. One challenge in designed cooperative catalysis is to access high catalytic proficiency (large enhancement in both rate and enantioselectivity) as seen in biocatalysis. Here described is an approach of developing and investigating trifunctional organocatalysts with three distinct catalytic functionalities, in order to understand how cooperativity could be organized for enantioselective activation that confers the observed proficiency in a tandem Michael-aldol-proton transfer elimination model reaction. This in future may assist in finding not just cooperative but also regulated catalysis to expand the level of catalytic complexity and efficiency in biomimetic systems.

Rate Accelerated Organocatalytic Ring-Opening Polymerization of L-Lactide via the Application of a Bis(thiourea) H-bond Donating Cocatalyst

A cocatalyst system consisting of an alkylamine base and a bis(thiourea) featuring a linear alkane tether is shown to dramatically increase the rate of ring-opening polymerization (ROP) of L-lactide versus previously disclosed monothiourea H-bond donors. Rate acceleration occurs regardless of the identity of the alkylamine cocatalyst, and the ROP remains controlled yielding poly(lactide) with narrow molecular weight distributions, predictable molecular weights and high selectivity for monomer. This H-bond mediated ROP of L-lactide constitutes a rare, clear example of rate acceleration with bis(thiourea) H-bond donors versus monothioureas, and the bis(thiourea) is shown to remain highly active for ROP at fractional percent catalyst loadings. Activation at a single monomer ester by both thiourea moieties is implicated as the source of rate acceleration.

Insights into the Morita–Baylis–Hillman reaction of isomeric dibenzofuran carbaldehydes: a theoretical and mass spectral study

Systematic theoretical and mass spectral investigations on the faster Morita–Baylis–Hillman (MBH) reaction of dibenzofuran-4-carbaldehyde (2) compared to its isomer, dibenzofuran-2-carbaldehyde (1) are described.

Synthesis of novel camphor-derived bifunctional thiourea organocatalysts

Synthesis and catalyst performance of 2,3- (types and ) and 2,8-disubstituted (type ) thiourea bifunctional organocatalysts was attempted. The synthesis of catalyst of type has, so far, not been realized, while catalysts of type , i.e., the 2,3-exo- and the 2-endo-3-exo-thiourea catalysts, were prepared in six steps starting from (+)-camphor. The catalysts of type were prepared from (+)-camphor in eight steps. All the potential catalysts as well as most of the intermediate products were carefully structurally characterized. The thiourea bifunctional organocatalysts were tested in a model reaction of Michael addition of dimethyl malonate to trans-β-nitrostyrene.

Synthesis and Structural Characterization of 1-Arylimidazole-2-thiones and N,N'-Aryldiethoxyethylthioureas with Electronically Diverse Substituents: A Manifold of Hydrogen Bonding Networks

The 1-arylimidazole-2-thiones, (HmimAr) [Ar = 3,4,5-C6H2(OMe)3, 2,4-C6H3(NO2)(OMe), 2,4,6-C6H2Cl3 and 3,5-C6H3(CF3)2], which feature electronically diverse substituents, may be obtained via acid-catalyzed ring closure of the corresponding N,N'-aryldiethoxyethylthiourea derivatives, ArN(H)C(S)N(H)CH2CH(OEt)2, (H2detuAr), which in turn are obtained via treatment of aminoacetaldehyde diethyl acetal, H2NCH2CH(OEt)2, with the respective arylisothiocyanates (ArNCS). The molecular structures of all of the above N,N'-aryldiethoxyethylthioureas and 1-arylimidazole-2-thiones have been determined by X-ray diffraction, thereby demonstrating that the substituents have a profound effect on the crystal structures. For example, each of the N,N'-aryldiethoxyethylthiourea derivatives adopts a different hydrogen bonding pattern. Specifically, the hydrogen-bonding network in (i) H2detuArCl3 consists of chains of 9-membered rings, with an [ [Formula: see text](9)] motif, that feature one N-H ⋯ O and one N-H ⋯ S interaction, (ii) H2detuArOMe,NO2 consists of chains of 6-membered rings, with an [ [Formula: see text](6)] motif, that feature two head-to-tail N-H ⋯ S interactions, (iii) H2detuAr(CF3)2 consists of a dimer that features two pairs of N-H ⋯ O interactions, of which each pair is a component of an 8-membered ring with an [ [Formula: see text](8)] motif, and (iv) H2detuAr(OMe)3 consists of a chain of head-to-head dimeric rings with a basic [ [Formula: see text](16)] motif, a notable feature of which is that sulfur does not play a role as a hydrogen bond acceptor. Each of the 1-arylimidazole-2-thiones exists as a "head-to-head" hydrogen-bonded dimer in the solid state, with an [ [Formula: see text](8)] motif. However, while the hydrogen-bonded motifs for HmimArCl3 and HmimAr(OMe)3 are planar, those for HmimAr(CF3)2 and HmimArOMe,NO2 are extremely puckered, with fold angles of 24.2° (mean value) and 45.7°, respectively.

The highly enantioselective catalytic aza-Morita–Baylis–Hillman reaction

Chiral phosphine, amine or metal catalyzed asymmetric aza-MBH reactions of various imines with different Michael acceptors in recent decades have been discussed and the perspectives offered by these new developments have also been demonstrated.

Synthesis of 2-(3-(3,5-bis(trifluoromethyl)phenyl)thioureido)-3-((dimethylamino)methyl)camphor organocatalysts

In a stereo-divergent synthesis, three novel camphor-derived bifunctional thiourea organocatalysts 7-9 have been prepared in five steps starting from (+)-camphor. In addition, borneol-derived bifunctional thiourea organocatalysts 19/19' have been prepared in three steps from (1S)-(+)-camphorquinone. Novel organocatalysts 7-9, 19/19' have been evaluated in a model reaction of Michael addition of dimethyl malonate to trans-β-nitrostyrene with low to moderate enantioselectivities (20%-60% ee). Configuration of all novel compounds has been meticulously determined using nuclear magnetic resonance (NMR) techniques.

Asymmetric organocatalyzed Morita-Baylis-Hillman reactions

The use of organocatalyst to promote the Morita-Baylis-Hillman (MBH) and aza-MBH reaction is an asymmetric and atom-economic way to obtain functionalized allylic alcohols or amines respectively, which have recently been well exploited, providing access to chiral densely functionalized products, which are valuable building blocks that can be further applied in the synthesis of complex natural products.

Synthesis and structural elucidation of novel camphor-derived thioureas

Nine novel (+)-camphor-derived thioureas have been prepared. 3-((Dimethylamino)methylene)camphor (2) served as the common precursor for the preparation of both, 2-thiourea 15-20 and 3-thiourea functionalized camphor derivatives 6, 7/7', respectively. Starting from 2, the latter were prepared in two or three steps whereas the former in five steps, respectively. Configuration of all novel compounds has been meticulously determined using NMR techniques and/or single crystal X-ray crystallography.

Organocatalytic asymmetric Michael addition of alpha-aryl cyclopentanones to nitroolefins for construction of adjacent quaternary and tertiary stereocenters

The first asymmetric Michael addition of alpha-aryl cyclopentanones to nitroolefins for construction of adjacent quaternary and tertiary stereocenters has been achieved with excellent diastereo-/enantioselectivity.